Mems microphone system for harsh environments

ABSTRACT

A MEMS microphone system suited for harsh environments. The system uses an integrated circuit package. A first, solid metal lid covers one face of a ceramic package base that includes a cavity, forming an acoustic chamber. The base includes an aperture through the opposing face of the base for receiving audio signals into the chamber. A MEMS microphone is attached within the chamber about the aperture. A filter covers the aperture opening in the opposing face of the base to prevent contaminants from entering the acoustic chamber. A second metal lid encloses the opposing face of the base and may attach the filter to this face of the base. The lids are electrically connected with vias forming a radio frequency interference shield. The ceramic base material is thermally matched to the silicon microphone material to allow operation over an extended temperature range.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.13/632,205, filed on Oct. 1, 2012, and entitled “MEMS Microphone Systemfor Harsh Environments”, the disclosure of which is incorporated hereinby reference, as though set forth in full.

TECHNICAL FIELD

The invention generally relates to microphones and, more particularly,the invention relates to packaged microphones for harsh environments

BACKGROUND ART

Microelectromechanical system (“MEMS”) condenser microphone diestypically have a diaphragm that forms a variable capacitor with anunderlying backplate. Receipt of an audio signal causes the diaphragm tovibrate and generate a variable capacitance signal representing theaudio signal. This variable capacitance signal can be amplified,recorded, or otherwise transmitted to another electronic device. Suchmicrophones typically are enclosed in a package to shield sensitivemicrophone components from environmental conditions.

These microphone packages have an aperture that allows acoustic signalsto reach the microphone. Undesirably, in addition to allowing access foracoustic signals, the aperture can allow access to contaminants that cancompromise microphone operation. For example, particles and moisture canenter the interior of the microphone and interfere with movement of thevariable capacitor. Additionally, thermal expansion mismatches betweenthe microphone die and the package materials on which the microphone dieis mounted can limit the operating temperature range for thesemicrophone systems. These problems often compromise condenser microphonesystems operating in harsh environments having very high-temperatures,radio frequency interference/electromagnetic interference (“RFI”/“EMI”),and high volumes of airborne contaminants such as dirt, dust, andliquids.

SUMMARY OF EMBODIMENTS OF THE INVENTION

In a first embodiment of the invention there is provided a silicon MEMSmicrophone system in an integrated circuit package for use in harshenvironments. The microphone system includes a package base whichincludes opposing first and second faces. The package base has a cavityin its first face and a first metal lid coupled to the first face of thepackage base covering the cavity to form an acoustic chamber. Thepackage base includes an aperture through the second face of the packagebase for receiving audio signals into the acoustic chamber. A MEMSmicrophone is secured to the package base about the aperture within theacoustic chamber. A filter covers the aperture in the second face of thepackage base to prevent contaminants from reaching the microphone. Asecond metal lid encloses the second face of the base and the filter;includes an opening for allowing the audio signal to reach the acousticchamber; and is electrically connected to the first metal lid, formingan EMI shield.

In some embodiments of the invention, the package base is a ceramicmaterial providing a good thermal expansion match for the siliconmicrophone and any silicon processing elements attached to the packagebase. In preferred embodiments, the metal lids are connected with viaswhose number and spacing reduces RFI that could affect microphonesignals. The filter material may be a fabric with a controlled acousticimpedance and may be chosen from a polyetheretherketone fabric or apolytetrafluoroethylene fabric or an equivalent, in preferredembodiments of the invention. The microphone components may communicatewith other system components external to the integrated circuit packageby various means including side-brazed in-line leads, J-leads, gull-wingleads or by surface mounting the integrated circuit package on a circuitboard, according to various embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of embodiments will be more readily understood byreference to the following detailed description, taken with reference tothe accompanying drawings, in which:

FIG. 1 schematically shows a cross-sectional view of a microphone systemconfigured in accordance with illustrative embodiments of the invention;

FIG. 2 schematically shows a cross-sectional view of the microphone inthe system of FIG. 1;

FIG. 3 schematically shows a top view of the microphone system of FIG.1, without the lid; and

FIG. 4 illustrates a process for fabricating the microphone system ofFIG. 1 in accordance with illustrative embodiments of the invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In an illustrative embodiment of the present invention, a microphonesystem is specially configured for use in harsh environments. Forexample, the microphone system should be capable of substantially normaloperation in unusual and extreme situations, such as those with extremetemperature swings, high humidity and moisture (e.g., nauticalenvironments), high shock (e.g., military environments), and very dryconditions (e.g., deserts). The system thus may be considered to be a“ruggedized” microphone system.

To that end, in summary, the microphone system may have a base with acavity, and a solid metal lid covering the cavity forming an acousticchamber containing a silicon microphone (i.e., a MEMS microphone). Thepackage base includes an aperture from the acoustic chamber through theopposing face of the base for receiving audio signals into the chamber.To prevent contaminants from entering the acoustic chamber, the systemalso includes a filter covering the aperture opening in the opposingface of the base. A second metal lid encloses the opposing face of thebase. To protect the silicon microphone from electromagneticinterference (a/k/a “EMI”), vias or other electrical interconnectorselectrically connect the lids to form an EMI shield (also referred to asan “RF interference shield”). Finally, the base material preferably isthermally matched to the silicon microphone to permit use across a widerange of temperatures. Details of various illustrative embodiments arepresented below.

FIG. 1 schematically shows a cross-sectional view of a microphone system10 according to an illustrative embodiment of the invention. As known bythose in the art, the microphone system (also known as a “packagedmicrophone”) may be mounted to a larger system, such as a printedcircuit board of a larger system. For example, the larger system may bea mobile telephone, and the printed circuit board may position themicrophone system 10 near an opening in its external housing to receivea voice signal. The printed circuit board also may have many componentsother than the microphone, such as power circuitry, filters, etc., thatmay or may not directly control the microphone system 10. In fact, thesystem printed circuit board may be a daughterboard that is secured to alarger printed circuit board (i.e., a motherboard).

The microphone system 10 shown may be considered to be a top port,cavity down design. This “top port” designation is due to the fact thatits port 90 (see below for details of the port) is on the side of thepackage that, when secured to the system printed circuit board, isfarthest from the board—it is on the top of the package from thatperspective. In a corresponding manner, the “cavity down” designation isdue to the fact that the cavity 40 (also discussed below) is concavefrom the perspective of the printed circuit board when mounted.

To those ends, the microphone system 10 includes a package base 20forming the noted cavity 40, and a solid metal lid 30 secured over thecavity 40. This connection of the lid 30 over the cavity 40 forms anacoustic chamber. The base 20 forms an aperture 50 through the opposingfaces for entry of an acoustic signal (e.g., a sound signal) into theacoustic chamber.

A silicon microphone 70 (i.e., a MEMS microphone) is attached to thefirst face of the base about the aperture 50. FIG. 2 schematically showsa cross-sectional view of the silicon microphone 70 in accordance withillustrative embodiments of the invention. Among other things, themicrophone 70 includes a static backplate 71 that supports and forms avariable capacitor with a flexible diaphragm 72. In illustrativeembodiments, the backplate 71 and diaphragm 72 each are formed fromsingle crystal silicon (e.g., the top layer of a silicon-on-insulatorwafer, discussed below). Alternatively, the diaphragm 72 is formed froma film of silicon-based material, such as polysilicon, silicon carbide,or silicon germanium. In a similar manner, other types of materials canform the backplate 71. For example, the backplate 71 can be formed froma relatively low temperature film, such as silicon germanium. If thermalbudget is not a primary issue, the backplate 71 can be formed from hightemperature materials, such as polysilicon, silicon carbide, or silicongermanium.

To facilitate operation, the backplate 71 has a plurality ofthrough-hole apertures (“backplate apertures 73”) that lead to abackside cavity 40. Springs 74 movably connect the diaphragm 72 to astatic/stationary portion of the microphone 70, which includes asubstrate that at least in part includes the backplate 71. The springs74 effectively form a plurality of openings that permit at least aportion of the audio signal to pass through the microphone 70. Theseopenings may be any reasonable shape, such as in the shape of a slot,round hole, or some irregular shape.

The aperture 50 thus forms a top port for the microphone system 10. Themicrophone system 10 thus has an enlarged back volume, which primarilyincludes the acoustic chamber formed by the cavity 40 and the lid 30. Asknown by those in the art, this enlarged back volume enhancesperformance of the entire system 10. Moreover, to prevent contaminantsfrom reaching sensitive microstructure forming the variable capacitor ofthe microphone, the aperture opening 50 preferably is covered by afilter 60. The filter should be selected to permit easy access todesired acoustic signals while mitigating the potential of access ofundesired particles.

A second metal lid 80 is attached to the opposing face of the base 20.The second metal lid includes an opening 90 to allow sound to reach theaperture 50 opening. In some embodiments, this second metal lid has araised portion that forms a front volume over the face to which it ismounted. Such a front volume is optional, however, and thus, may beomitted.

In illustrative embodiments, the first and second metal lids 30 and 80are electrically connected to form an EMI shield about the microphonedie 70. The two metal lids 30 and 80 at least in part form a Faradaycage that should mitigate the impact of EMI. Accordingly, the microphonesystem 10 has an electrical interconnection between the two lids 30 and80, ensuring they are at substantially the same electric potential. Tothat end, the system shown in FIG. 1 has a plurality of vias 85, formingan enclosure that reduces EMI that could inject noise into microphonesignals.

In some embodiments, the microphone system 10 has a specialized circuitdie 110 that controls operation of the microphone die 70. To that end,the die 100 may be implemented as an application specific integratedcircuit (“ASIC 110”) attached to the first face of the package base 20(i.e., within the cavity 40). Wire bonds 120 connect this circuitelement die 110 to the microphone 70, and one or both of the microphonedie 70 and ASIC 110 to pads on the base 20. FIG. 3, a top down view ofthe microphone system without the package lid 80, shows the pads 115.The pads 115 also attach the wire bonds 120 (and, consequently themicrophone 70 and ASIC 110) to the interconnects (discussed below) thatconnect the system 10 to the noted system printed circuit board.

FIG. 2 also shows vias 85 that connect the metal lids 30, 80. The numberand spacing of the vias 85 can be chosen such that EMI can be suppressedby the combination of the two metal lids and the vias.

The microphone system 10 can use any of a wide variety of electricalinterconnection systems to electrically and mechanically connect with anunderlying system circuit board or base. For example, FIG. 1 shows aplurality of leads 100 extending from the base to allow the microphonedie 70 (and ASIC die 110, described below) to communicate with otherelectronic elements (not shown). As shown in FIG. 1, such leads 100 maybe side brazed in-line leads for connecting directly to a circuit board(or to a socket). Alternatively, the leads may be J-leads or gull-wingleads (not shown) for surface mounting the microphone system package toa circuit board. In yet other embodiments, the system 10 can have pads(not shown) for surface mounting with the underlying system.

The package base 20 material may be chosen so that its coefficient ofthermal expansion (“CTE”) closely matches the CTE of the microphone 70and that of the ASIC 110. Accordingly, mitigating the CTE mismatchshould mitigate thermal stress between the base 20 and microphone systemelements, favorably expanding the operating temperature range of theoverall microphone system. In preferred embodiments, the package basematerial is a ceramic, which has a CTE that is close to that of silicon.The ceramic material for the base may be chosen from a variety ofceramic materials including, by way of example but not for limitation,aluminum nitride and aluminum oxide (alumina). The ceramic package basemay be fabricated by a high temperature co-fired ceramic process, by alow temperature co-fired ceramic process, or by another process as isknown in the art.

In other embodiments of the invention, materials other than ceramics maybe used for the package base, such as a plastic or a composite material,e.g., FR-4 epoxy laminate. In preferred embodiments of the inventionusing a ceramic package base, the operating temperature range of thesemicrophone systems can extend from about −55 degrees C. to about 215degrees C. and beyond.

The above noted filter 60 should seal the acoustic chamber from manycontaminants. To that end, the filter can be attached to the opposingface of the package base 20 with adhesive, such as, for example, anepoxy adhesive. In some embodiments of the invention, the filter isattached to the package base by a form-fitting second metal lid 80,either with or without adhesive applied to the package base. The filtermaterial may be chosen from any of the high temperature acoustic fabricsthat are known in the art. These fabrics provide a controlled acousticimpedance and may include, among other things, polyetheretherketone(“PEEK”) fabric, polytetrafluoroethylene (“PTFE”) fabric, or theirequivalents. The filter may also be formed in the second metal lid 80.For example, the filter may be formed by sintering. The filter may alsobe formed in the package base, for example, by sintering.

It should be noted that discussion of a top port, cavity down package isbut one of a number of different potential embodiments. For example,some embodiments may use a bottom port design, with the cavity either upor down. As another example, some embodiments may have a cavity updesign with either a top or bottom port design. Moreover, rather thansolid metal lids 30 and 80, some embodiments may be formed from anothermaterial, or a composite, layered set of materials. For example, someembodiments may use metal plated lids, conductive or nonconductiveplastic lids, or plastic lids with metal layers. The type of lid andconfiguration depends on the intended application.

In fact, some embodiments may omit one of the lids 30 and 80.Alternatively, other embodiments may use one lid formed from one set ofmaterials (e.g., a solid metal lid) and another lid 30 or 80 formed fromanother set of materials (e.g., a plastic lid). Accordingly, discussionof solid metal lids 30 and 80 is for illustrative purposes only and isnot intended to limit all embodiments.

Fabrication Process for MEMS Microphone System

FIG. 4 shows a process of fabricating the microphone system 10 inaccordance with illustrative embodiments of the invention. It should benoted that for simplicity, this described process is a significantlysimplified version of an actual process used to form the microphonesystem 10. Accordingly, those skilled in the art would understand thatthe process may have additional steps not explicitly shown in FIG. 4.Moreover, some of the steps may be performed in a different order thanthat shown, or at substantially the same time. Those skilled in the artshould be capable of modifying the process to suit their particularrequirements.

The process begins at step 210, which provides a package base 20 havingopposing first and second faces with the base. As shown in FIG. 1 anddiscussed above, the base has the cavity 40 in the first face, vias 85,and the aperture 50 through its second face. The process then securesboth 1) the MEMS microphone 70 in the cavity about the package baseaperture (step 220), and 2) the ASIC 110 next to the microphone in thesame cavity. Among other ways, the process may use a conductive orinsulative adhesive to secure bottom sides of both chips to the cavity.Wire bonds 120 may be placed to connect ASIC 110 to the microphone 70,and one or both of the microphone die 70 and ASIC 110 to pads 115 on thebase 20.

Next, the process attaches the first metal lid 30 (step 230) to thefirst face of the package base 20 to form the acoustic chamber, whichcontains the microphone. In addition, this step also electrically andmechanically connects the first metal lid 30 with the vias 80. A filter60 then is attached (step 240) to the opposing face of the package base,covering the aperture 50. The process then may secure second metal lid80 (step 250) to the opposing face of the base in a manner thatmechanically and electrically connects with the vias (step 260).Accordingly, this also electrically connects both lids, forming the EMIshield. Among other ways, the lids may be attached to the package baseby low temperature soldering using solder such as a tin-silver-copperalloy. Alternatively, attachment of the lids may be accomplished withseam welding, or by using a conductive adhesive.

Accordingly, illustrative embodiments permit use of a MEMS microphone ina wider variety of environments. The CTE matched base and dies enables awider temperature variation, while the opposing lids provide EMIprotection and a more robust package. Moreover, the filter furtherlimits particle access to the sensitive microstructure of the MEMSvariable capacitor

The embodiments of the invention described above are intended to bemerely exemplary; numerous variations and modifications will be apparentto those skilled in the art. All such variations and modifications areintended to be within the scope of the present invention as defined inany appended claims.

What is claimed is:
 1. A method of forming a microphone system, themethod comprising: providing a package base including opposing first andsecond faces, the base including a cavity in the first face, vias, andan aperture through the second face; securing a MEMS microphone aboutthe package base aperture; securing a first metal lid to the first faceof the package base to form an acoustic chamber including the cavity,wherein the acoustic chamber contains the microphone; attaching a filterto the second face of the package base wherein the filter covers theaperture; and attaching a second metal lid to the second face of thebase; and electrically connecting the vias to each of the first metallid and the second metal lid.
 2. The method of claim 1 wherein thepackage base is ceramic.
 3. The method of claim 1 wherein the first andsecond metal lids are attached to the package base by seam welding. 4.The method of claim 1 wherein the first and second metal lids areattached to the package base by soldering.